1. Field of the Invention.
This invention relates to data modem receivers of the kind for receiving analog signals transmitted on a transmission line from a remote location, said analog signals including a training signal having a spectrum with strong components at first and second predetermined frequencies, and analog-to-digital conversion means adapted to convert said analog signals to corresponding digital signals.
2. Description of the Prior Art.
In a multipoint data modem network, a master station polls several remote stations. A multipoint data modem communication system is known from European patent application No. 0,169,548. In this known system, the modem of the master station gives a continuous carrier signal which is received by each remote modem. If the terminal identity contained in the poll does not match the identity of the remote station then the poll is ignored. A remote station that recognizes the poll responds by sending one of two types of "messages"; either the first block of its data or a character sequence indicating no data to send. A remote station must use switched carrier signals when sending its "message" by turning on "request to send" to its modem before transmission and turning it off after transmission. Once a particular remote station becomes active, the master station modem must detect the carrier of the remote modem, extract the receiver timing signal, set internal gain parameters and acquire equalizer values to compensate for the transmission path to the active modem. Each modem communication channel has a separate and distinct path resulting in a different set of characteristics corresponding to attenuation, amplitude and delay distortion and phase impairments. Therefore, the master station modem must compensate for each communication channel individually. Each remotemaster communication channel requires the connected modem to send a training sequence to allow the master modem receiver to acquire the parameter required for reliable data transfer. For that purpose the master modem acquires receiver operating parameters and coefficients in a memory location allocated for the particular remote modem. The stored parameters include parameters for compensation of data timing clock offset and equalizer coefficients. At the beginning of a subsequent transmission the master modem receiver parameters and coefficients are loaded from values previously stored in the corresponding memory location. During subsequent transmissions from a remote modem, the modem is identified at the master modem by a time interval or by a set of frequencies. During short remote-master transmissions each training sequence occupies a significant part of the transmission time. It is therefore desirable to minimize the duration of the training sequence.
In a known data modem receiver of the kind specified, disclosed in U.S. Pat. No. 4,577,334, the start of an incoming line signal is detected by an energy detection circuit which operates when the received signal energy level exceeds a predetermined threshold. This has the disadvantage that the start of an incoming signal might be detected in response to noise or a secondary channel signal, rather than a valid training signal.